1. Field of the Invention
This invention relates to rocket motor ablative materials, especially resin-filled carbon fiber and carbon/carbon ablative materials, and a method of making the ablative materials. In particular this invention relates to carbon ablative materials having a reinforcement component formed from, as a precursor prior to carbonization, carded and yarn-spun staple cellulosic fibers. This invention also relates to rocket motor assemblies including the carbon ablative materials.
2. Description of the Related Art
It is generally accepted current industry practice to prepare insulation for solid propellant rocket motors from a polymeric base composite importantly including a carbon cloth. The composite is generally composed of the carbon cloth as a woven reinforcement structure impregnated with a suitable resin matrix. The resin matrix is commonly a phenolic resin, although other resin matrices can be used. For making the woven reinforcement structure, current industry practice is to select a continuous filament non-solvent spun viscose rayon as a precursor material. The continuous filament viscose rayon, which is especially formulated for ablative applications, is woven, wound, or otherwise manipulated into its desired configuration and then carbonized to form a carbon structure exhibiting superior ablation characteristics and excellent physical properties and processability.
Continuous filament viscose rayon precursor has been established as a standard in the rocket motor industry for making carbon reinforced structures of carbon and carbon/carbon ablative materials due to its superior ablation characteristics, excellent physical and thermal properties, and high processability. One of the excellent physical properties possessed by composites formed from continuous filament viscose rayon precursor is a cured composite high warp strength of about 144.8 MPa (or about 21,000 lbs/in.sup.2) at ambient temperature (about 21.degree. C. or 70.degree. F.), as measured subsequent to carbonization and impregnation of the precursor. Warp strength reflects the tolerance of the filament to opposing forces acting along the warp (or longitudinal) filament axis.
However, a major drawback associated with the use of cured composites comprising wrapped layers of continuous filament viscose rayon, such as found within the bulk areas of much rocket nozzle insulation, is the relative low across-ply tensile strength possessed by the carbonized continuous filament viscose rayon at operating temperatures experienced within the bulk ablative material (as opposed to the exhaust gas surface) during firing of a rocket motor. Such firing temperatures within the bulk ablative material generally can rise to about 400.degree. C. (or 750.degree. F.). Specifically, cured composites comprising wrapped layers of carbonized continuous filament viscose rayon have across-ply tensile strengths on the order of about 2.07 MPa (or about 300 lbs/in.sup.2). As referred to herein, across-ply tensile strength is the amount of load, perpendicular to the filament axes, which two overlapping layers of filaments can withstand prior to slippage.
Another significant drawback associated with continuous filament viscose rayon that has recently drawn significant attention involves the availability of this particular type of continuous filament. Over the past few years, the only manufacturer producing sufficient quantities of continuous filament viscose rayon to meet industry demands is North American Rayon Corp. (NARC) of Elizabethton, Tenn. The capability of the industry to produce ablative liners and other thermal insulation based on continuous filament viscose rayon has been jeopardized, however, due to the cessation of continuous filament viscose fiber production by NARC. There is therefore a need in this industry, previously not satisfied, to find an effective alternate source or a replacement candidate for the above-described standard thermal insulation formed from continuous filament viscose rayon precursor.
The requirements that a replacement candidate must satisfy in order to be acceptable and functionally effective are well known to be quite severe due to the extreme conditions to which the insulation is exposed. These conditions not only include exceedingly high temperatures but also severe ablative effects from the hot particles (as well as gases) that traverse and exit the rocket motor interior, or over the outer surface of re-entry vehicle insulators. Unless the insulation will withstand such conditions, catastrophic failure may (and has) occurred.
Accordingly, any replacement insulation should exhibit comparable temperature resistant and ablation characteristics and rheological and physical properties at least equivalent to those of continuous rayon viscose filament, yet should not otherwise significantly alter the manufacturing process employed for the production of the thermal insulation. Additionally, due to the large and growing quantities of solid propellant rocket motor insulation required by the industry, any such replacement reinforcement precursor candidate should be abundantly available now and into the foreseeable future.
An alternative carbon precursor that has been proposed for ablative applications is continuous filament polyacrylonitrile (PAN). PAN continuous filaments disadvantageously possess higher densities than cellulosic materials (1.8 g/cm.sup.3 for PAN, compared to 1.48 g/cm.sup.3 for cellulosic filaments) and higher thermal conductivities than cellulosic materials. Thus, in order to provide a comparable insulation performance to rayon filaments, rocket motor nozzle insulation or re-entry vehicle insulation formed from PAN filament must have a greater thickness and weight than a comparable-performing insulation formed from cellulosic materials. The replacement material must meet the ablation limits for protection of the casing (when used as an internal casing insulation) throughout the propellant burn without adding undue weight to the motor.
Accordingly, the search for a functionally satisfactory precursor for making the reinforcement structure of a composite material requires discovery and implementation of an extraordinarily complex combination of characteristics. The criticality of the material selection is further demonstrated by the severity and magnitude of the risk of failure. Most insulation is of necessity "man-rated" in the sense that a catastrophic failure can result in the loss of human life--whether the rocket motor is used as a booster for launch of a rocket or is carried tactically underneath the wing of an attack aircraft. The monetary cost of failure in satellite launches is well-publicized and can run into the hundreds of millions of dollars.
Therefore, one of the most difficult tasks in the solid propellant rocket motor industry is the development of a suitable, acceptable insulation that will meet and pass a large number of test criteria to lead to its acceptability.
Furthermore, any replacement precursor should not be susceptible to obsolescence issues nor discontinuance in future supply thereof.